1. Field of the Invention
The present invention relates in general to gas separation devices of the gas chromatographic variety and in particular, relates to improved devices for conducting gas chromatographic separation of components of volatile sample mixtures, utilizing elevated column pressure, controlled column flow rate including intermittent stop flow operation and improved means for contamination free handling and concentration of trace quantities of separated sample components.
2. Description or Prior Art
Gas chromatograph instruments comprise a class of extremely sensitive devices for the separation of components for a volatile sample mixture. Usual practice has been to mix the volatile sample with an inert carrier gas, the mixture of which is then percolated through a column containing granulated particles providing a large surface area. Depending upon the choice of the granulated packing material, a solid or liquid stationary phase is interfaced with the mobil carrier and sample mixture gaseous phase. The sample mixture components are, in the percolating process, partitioned between the mobil gaseous phase and the stationary liquid or solid phase. Each component or, if poorly resolved by the process, each class of component compounds will exhibit a unique rate of travel through a given column referred to as the retention time for the compound in the column.
Heretofore, extensive investigative work has been conducted with temperature programming of chromatographic columns and with various packing materials for columns with a purpose to increase solubility of the sample mixture in one or both the stationary phase and mobil gaseous phase. Comparable investigation work has not been conducted to date to examine the effect of increased low and intermedial range column exit pressure, i.e., gauge pressures in the range of zero to fifty atmospheres on the efficiency and operation of chromatograph columns. Some earlier work at high pressures, i.e., pressures in the 1000 to 2000 atmospheres (absolute) range, has been conducted to separate large molecular weight molecules. The earlier very high pressure investigations depended upon the altered near liquid like density of the carrier gas at extreme pressures to heighten sample solubility in the gaseous phase and facilitate separation of high molecular weight compounds. From the aforesaid high pressure investigations no readily useable laboratory device for chromatographic procedures in the pressure range of one to 50 atmospheres absolute was disclosed.
Usual practice has been to operate conventional gas chromatographs at the column gas velocity which optimizes the resolution and speed of operation for a specific sample mixture. This procedure has been achieved by operating the output opening of the column at atmospheric pressure, and regulating the flow rate of carrier gas injected into the column input opening from a pressurized tank. Commonly, the pressurized cylindrical tank reservoir of carrier gas is held at pressures above 200 atmospheres. In conventional gas chromatograph laboratory practice during operation, the pressure within the column in the vicinity of the carrier gas input opening is between one and three atmospheres gauge pressure. By adjusting the carrier gas input flow rate, the carrier gas velocity within a specific column may be adjusted to achieve the resolution and sample separation desired. The pressure drop between the input and output regions of conventional chromatograph column usage ranges between 1 and 3 atmospheres. Accordingly, the gas velocity in the column is substantial due to the pressure drop forces along the length of the column. Under these conditions, any sudden change in the gas pressure or gas velocity at the column outlet opening, such as may be induced in conventional sample collection procedures, may produce mixing within the column and reduce the resolution and separation of following separated but still entrained components.
Conventional gas chromatograph laboratory practice, in order to achieve rapid highly resolved separations, utilizes higher as distinct from lower carrier gas velocities within the column to reduce time the purified sample entrained in the lower portion of the column may be exposed to diffusion. Only with relatively high column carrier gas velocities can laminar flow through a substantial portion of the column and less mixing in the lower portions of the column in the vicinity of the column output opening be achieved under conventional column pressure ranges.
The high gas velocity in the chromatograph column causes some difficulty when the emergent separated components must be confined in a container for later analysis or use. Separated and entrained sample components within the column moving at rapid velocities "pile up" if the column is stopped or restricted in a transient manner for purposes of segregating a sample component as it leaves the column. Moreover, in chromatograph columns as conventionally operated, a pressure of approximately one atmosphere (absolute) prevails in the vicinity of the outlet opening. When the column is operated in a stop flow mode, the rate of gaseous diffusion is sufficiently high at the relatively low one atmosphere (absolute) exit pressure that the separated and entrained components of the sample mixture held in the lower portion of the column are rapidly mixed by diffusion and the resolution or separation between them is degraded. That is, a separate and pure sample component becomes contaminated with diffusion of other components of the sample from within the column.
In continuous line systems, a sample chamber, as might be utilized in spectral analysis, of separated sample components is preferably securely sealed to the output aperature of the chromatograph column. By such an arrangement, risk of contamination of a separated pure compound from outside the chromatograph column is significantly reduced. However, in such arrangement, the time for cycling the spectral analysis procedure must be coordinated with the difference in retention times of various sample components leaving the chromatograph column. Spectral analysis procedures such as Infrared Spectroscopy normally require more time than the difference in retention times of several separated compounds passing through a chromatograph column. The same condition characterizes other spectral analysis methods such as, for instance, mass spectrometry, ultra violet and visible band spectrometry, Raman spectrometry, and nuclear magnetic resonance; also a similar condition characterizes analytic procedures that are not spectral, such as electromechanical polarographic and coulometric analysis. Present laboratory practice reconciles the differences, on the one hand of the time of response of chromatographic separation processes, and on the other hand, the normally much slower response time of spectral and certain other analysis processes by one of two methods, neither of which is completely satisfactory.
First, some spectral analysis are done on the fly. The fly scan method, necessarily only one rapid scan, fails to extract the optimum spectral analysis data from the moving purified sample. Valuable data is often lost. The fly scan method, however, avoids disturbing the gas flow emitted from the column outlet opening and the attendant mixing in the gas stream due to pressure disturbance that may be reflected back into the column which stop flow operation in conventional existing devices would certainly cause.
The second method in current practice is that of storing in a detachable container a pure sample of the eluted materials issuing from the outlet of the chromatograph column. This latter approach minimizes but does not avoid the disturbance of gas flow in the column with its attendant mixing within the operating column. It may expose the pure samples to outside contamination sources. When the sample is stored, it is possible to scan repeatedly with the spectral analysis device and obtain the optimal spectral analysis data.
Conventional practice furnishes the purified sample to the storage chamber at a pressure of one atmosphere. Some spectral analysis procedures, such as infrared spectroscopy requires that the sample be concentrated before analysis, thus requiring still another step with the attendant added costs, time and contamination risks.
The most convenient manner of achieving the required time coordination between a chromatograph and a spectral analysis device is to operate the chromatograph column in a stop flow mode, provided this can be achieved without reducing the resolution and separation of subsequent entrained components.
Heretofore, no chromatograph device has been available which provided low velocity control of the carrier gas moving through the column while superior resolution of separated sample components was maintained. Similarly, heretofore, no chromatograph device has been available to this time, in which stop flow mode of operation could be conducted and good separation of eluted pure materials maintained.
Finally, an in line chamber attached to the outlet of a chromatograph column, which chamber being for the purpose of retaining sample component compounds for additional analysis, is subject to contamination by lingering traces of earlier separated samples which had previously entered the chamber from the chromatograph column. Present in line sampling equipment for continuous use, at the chromatograph output does not provide convenient or certain means for insuring that no instrumentally detectable traces of sample compounds will be retained in the spectral analysis chamber after completion of the spectral analysis operations.